Nicotine mimics a chemical needed to make nerves function - thus it brings a chemically induced "rush." Then the body adapts and makes less of the natural chemical. Essentially, natural chemical plus nicotine then equals the level a non-smoker has. To get another rush the smoker would then need to increase the nicotine, to smoke more... But... their body adapts again and reduces the amount of the natural chemical. If the smoker stops smoking or reduces consumption, their nerves cease to function normally and they feel "jittery," they no longer make enough of the natural chemical on their own. Thus smoking is the most pointless of traps. The smoker spends all that money with no gains, gets messed up lungs, an increased chance of cancer, plus an addiction. To wean themselves from it is the only smart move.